Thermo-expansive microspheres comprising thermoplastic resinous shell and a blowing agent encapsulated in the shell are generally called thermo-expansive microcapsules. Various processes for producing thermo-expansive microcapsules have been studied. Japanese Patent Publication Sho 42-26524 describes a general process for producing thermo-expansive microcapsules. U.S. Pat. No. 3,615,972 describes a production process of thermo-expansive microspheres having polymeric shell formed into uniform thickness.
For producing thermo-expansive microcapsules, hydrocarbons, for example, n-butane, isobutane, isopentane and neopentane, are usually applied. Above all, isobutane and isopentane, which impart superior expanding performance to thermo-expansive microcapsules, are used.
MATSUMOTO MICROSPHERE (produced by Matsumoto Yushi-Seiyaku Co., Ltd.), a commercially available product of thermo-expansive microcapsules, comprises thermoplastic resin, such as vinylidene chloride polymer, acrylonitrile copolymer and acrylic polymer, in which blowing agents, such as isobutane and isopentane, are encapsulated.
Thermo-expansive microcapsules comprising polymers containing a considerable amount of chloride are flame-retardant even they contain flammable gases. However, they are hazardous because they generate chloride and hydrogen chloride gases, and further generate phosgene when they are ignited.
Employing a blowing agent other than flammable hydrocarbons for imparting flame resistance to thermo-expansive microcapsules has already been proposed. U.S. Pat. No. 3,615,972 discloses certain chlorofluorocarbons applicable for the purpose, though they have not been applied in commercial production. Chlorofluorocarbons do not impart sufficient expanding performance to thermo-expansive microcapsules, and they have other shortages.
Flon has also been applied for various flame-resistant products owing to its unique property. Although flon gas was admitted to be inert and have been used for a long time, recently, as popularly known, the use of flon gas has been restricted since the depletion of ozone shield became a serious problem, and applicable fluoro-compounds are being reexamined.
Actually, chlorofluorocarbon has been replaced by chloride-free aliphatic fluorocarbons or fluorohydrocarbons. Microcapsules produced of those chloride-free fluoro-compounds are disclosed in Japanese Patent Laid Open Hei 6-49260.
Aliphatic fluorocarbons or fluorohydrocarbons are inert indeed and have low ozone-depleting potential. However, they cannot be applied for producing thermo-expansive microspheres in the form of mixture with hydrocarbons of sufficient quantity for imparting high expanding performance to thermo-expansive microspheres, because their molecules, in which hydrogen of hydrocarbon moieties was only substituted with fluorine, have poor polarity and compatibility to hydrocarbons. Application of only one aliphatic fluorocarbon or fluorohydrocarbon causes a serious problem, i.e., insufficient expanding performance of resultant microspheres, because such fluorocarbon cannot be completely encapsulated in thermoplastic resinous shell in polymerization reaction due to their poor compatibility to monomers, and results in the formation of microspheres having thermo-plastic resinous shell impregnated with the fluorocarbon.
PCT International Application nationalized and published in Japan No. 2002-511900 discloses thermo-expansive hollow particles, being filled with an expanding agent of a mixture of (a) fluoro-hydrocarbon fluid and (b) organic ester, ether or ketone. Although fluoro-hydrocarbon fluids, such as aliphatic fluorocarbons or fluorohydrocarbons, are inert and have low ozone-depleting potential as described above, they are not preferable because of their high global warming potential. Aliphatic fluorocarbons or fluorohydrocarbons with low fluorine-substitution degree are not preferable, even if they are compatible to monomers, because resultant thermo-expansive hollow particles exhibit flammability. The particle sizes of the thermo-expansive hollow particles produced of those fluorine compounds distribute in a broad range, for example, a distribution range with a CV or coefficient of variation greater than 30%, which causes difficulty in providing products of constant expanding performance.
In the examples 11 and 12 of the nationalized and published patent application, thermo-expansive hollow particles produced of (a) fluorohydrocarbon fluid, such as the mixture of 1,1,1,2,3,4,4,5,5,5-decafluoropentane and perfluorohexane (PF-5060), and (b) one of organic esters, ethers and ketones, such as dimethyl hexafluoroglutarate and dimethyl octafluoroadipate are described as examples. The hollow particles have low expanding capacity and their particle size distributes in a broad range.
With those reasons, expansive microcapsules are not commercially and popularly available at present.